Currently, 3GPP is considering development of E-UTRA and E-UTRAN as set out in the technical specification 3GPP TS 36.300 v 8.1.0 (2007-06), incorporated herein by way of reference, and related documents. 3GPP Long Term Evolution (LTE) aims to enhance the Universal Mobile Telecommunications System (UMTS) standard, for example, by improving efficiency and services.
In E-UTRAN, user equipment (UE) communicates with a network node, NodeB (eNB), with data being sent on radio bearers (RBs) over a radio link between them. The eNB interfaces with a Mobile Management Entity (MME) via an interface designated as S1. The E-UTRAN network includes a plurality of eNBs and MMEs. The connection between the UE and the source eNB to which it is attached may be lost, this being known as radio link failure (RLF). It had previously been proposed that a UE experiencing an RLF and reappearing in a cell of another eNB would be handled as a transition via an idle state, RRC_IDLE. In this mechanism, when there is RLF in the source eNB, the UE enters IDLE state and selects a new target eNB. When the UE has attached to the new target eNB, and received the system information for the target eNB, the UE may perform the access procedure to establish resources in the target eNB. However, this procedure is time-consuming.
RLF may be particularly likely during fading channel conditions, when handover from the source eNB to a target eNB may be imminent. To avoid going via the IDLE state, it has been suggested that a new eNB could be prepared beforehand to accept the UE by using the handover preparation procedure. The Handover_Request message can be sent to multiple eNBs which are thus able to recognize the UE, each of the eNBs being prepared as handover candidates. Then, the UE is able to carry on with its old context after having chosen the new target eNB during the mobility phase of the RLF.
The paper by Nokia Siemens Networks et al “Radio Link Failure Recovery”, R2-072382, 3GPP TSG-RAN WG2 Meeting #58, incorporated herein by way of reference, discusses RLF recovery by preparing multiple eNBs using the handover procedure. A prepared eNB has the UE context and resources reserved, so by preparing multiple eNBs, the probability is increased that the UE can recover from RLF without needing to go via RRC_IDLE, thus reducing recovery time.
The paper by Nokia and NSN “Handover Failure Recovery” R2-071717, 3GPP TSG-RAN WG2 Meeting #58, incorporated herein by way of reference, and the paper by Vodafone “Mobility in LTE_ACTIVE state, R2-0723823, GPP TSG-RAN WG2 Meeting #58, incorporated herein by way of reference, also consider the preparation of multiple eNBs during handover to avoid the UE going via the IDLE state.